EDMONTON — Each winter, the Canadian Army rolls howitzers onto the Trans-Canada highway and shoots hundreds of explosive rounds up into the Selkirk Mountains in B.C., triggering safe, controlled avalanches and preventing dangerous natural ones.

But some of those rounds don’t explode — landing with a thud, not a bang.

That’s when a military bomb disposal unit is called in to safely destroy the unexploded shell.

Since the winter of 1997-98, roughly 13,300 explosive shells have been fired up into the mountains along the Rogers Pass, a shortcut for the Trans-Canada Highway and the Canadian Pacific Railway. In that same 20-year period, roughly 77 shots didn’t explode, according to data provided by Parks Canada.

It’s hard to say how many of those projectiles are still out there, but Parks Canada said that 50 of them have been destroyed since the 1960s.

The unexploded shells aren’t an enormous danger — they are often located in remote areas, away from trails or other visitor facilities — but Parks Canada still tries to track them down.

“Parks Canada tracks every 105 mm round fired, including date and time fired, target location, and whether it detonated or not,” Parks Canada spokesperson Allison Fleischer said in an email. “Each year Parks Canada searches all areas where unexploded ordnances (UXO) have been tracked. If a UXO is found, a DND Explosive Ordinance Disposal team is called to safely destroy the unexploded round.”

In August, a disposal team was dropped by helicopter, hiked in, and found and destroyed six unexploded shells. (The Post asked to accompany a bomb disposal unit on such a journey, but our request was declined.)

“Recovery and disposal … depends on their location, some areas are inaccessible and the terrain too dangerous to put people in to search or do explosive ordnance disposal operations,” Parks Canada spokesperson Shelley Bird said an email.

The Post reached out to several tour and mountaineering organizations who operate in the region, and none of those who replied had ever even seen unexploded ordnance while out in the backcountry. (Still, the entire area is closed or access is restricted to the public during the winter months for safety reasons.)

The Royal Canadian Artillery usually starts waging battle against Mother Nature by late November. The combat arms branch of the army fires 105 mm rounds from 17 different positions along Rogers Pass, a part of the country where some 12 metres of snow falls each year. The rounds are fired between three and five kilometres up into the mountains and the idea is to trigger avalanches before they get large enough to cause a tragedy. With more than 130 avalanche paths that cross the Trans-Canada Highway, Rogers Pass has the highest avalanche rating of any major road in North America, according to the Canadian Forces. In 1910, an avalanche killed 58 railway workers, and more than 250 died between 1885 and 1916.

If the controlled avalanches reach the highway, they are plowed out of the way. With about 4,000 vehicles trundling through the Rogers Pass per day, the Canadian Forces cite the economic impact of a road closure as a reason to keep the operation running in tip-top shape.

The overall failure rate is less than one per cent, most of the time, though some years are worse than others: A failure rate of 2.3 per cent in 2007-08 meant that 16 shells went unexploded out of 701 fired. Contrast that with the busiest shooting year in the last two decades, 2011-12, when 1,511 shells were fired, but a failure rate of less than half a per cent meant only five went unexploded.

With snow already falling in parts of the mountains, it’s only a matter of time before Canada’s military is back out there.

They’ll be fighting what Parks Canada calls “the snow war” — a battle that “is the story of the human struggle with nature in Rogers Pass.”

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